Double-walled vacuum insulated containers for the storage of cryogenic liquids are well known in the art. Containers of this type may be used, for example, to hold a liquid nitrogen refrigerant and function as a refrigeration unit for the storage of biological materials. Containers of this type are also used for the storage and dispensing of liquid oxygen for vaporization and use as breathing oxygen. Such containers are useful for a wide variety of purposes which require the storage and/or dispensing of a cryogenic liquid. Cryogenic liquids are generally considered to be liquids having a low boiling point at one standard atmosphere (760 mm Hg) with common examples being liquid nitrogen (77.degree. K.), liquid oxygen (90.degree. K.), liquid argon (87.degree. K.), liquid hydrogen (21.degree. K.), liquid helium (5.degree. K.) and liquid air (80.degree. K.).
Since cryogenic liquids have a very low heat of vaporation, even small quantities of thermal energy flowing from ambient into the liquid cryogen cause significant losses of cryogen through evaporation.
The prior art has thus devoted considerable effort in the design of double-walled vacuum insulated cryogenic liquid storage containers to minimize losses of cryogen due to heat inflow from the ambient. Accordingly, much progress has been made in the development of high performance container insulation systems to reduce the heat inflow from the ambient through the container insulation to the stored cryogen. These developments include:
Containers have been developed having evacuable insulation spaces capable of attaining and maintaining very low pressures on the order of less than 0.1 microns Hg when holding a cryogen.
Composite multilayered thermal insulations have been developed comprising radiation barrier materials interleaved with low heat conductive materials. Thermal insulations of this type are described, for example, in U.S. Pat. No. 3,007,596-Matsch, U.S. Pat. No. 3,009,600-Matsch; U.S. Pat. No. 3,006,601-Matsch; U.S. Pat. No. 3,145,515-Clapsadle; U.S. Pat. No. 3,265,236-Gibbon et al.; U.S. Pat. No. 4,055,268-Barthel; and U.S. Pat. No. 4,154,363-Barthel. An example of a composite multi-layered thermal insulation having one component comprising a metal coated organic plastic film is described in U.S. Pat. No. 3,018,016-Hnilicka.
A further prior art development in cryogenic storage container insulation systems is to dispose heat exchanger shields within the multilayered insulation and connect these shields to the neck tube of the container to conduct part of the thermal energy inflowing through the insulation through the neck tube wall into the cold effluent gas which carries it away to the surrounding atmosphere. Such an insulation system is described, for example, in U.S. Pat. Nos. 3,133,422-Paivanas et al. and 3,341,052-Barthel.
A recent improvement in cryogenic storage container insulation systems is to dispose a high thermally conductive member within the insulation for intercepting inflowing heat which is thermally joined to a thermal electric heat pump positioned within the insulation wherein the thermoelectric heat pump rejects the intercepted heat back to the ambient. Such a system is described in U.S. patent application Ser. No. 96,407 issued as U.S. Pat. No. 4,287,720 to Barthel.
Thus, the prior art has made many improvements in cryogenic container thermal insulation systems to reduce heat inflowing from the ambient into the liquid cryogen. However, thermal insulations and their optimization as a system are approaching the limit of efficiency beyond which further improvements result in negligible advantage or are not economically feasible.
Nevertheless, further reductions in heat inflowing from the ambient into the liquid cryogen stored in a cryogenic container is of great interest to the art.
Another path of heat inflow from the ambient into a liquid cryogen stored in a double-walled cryogenic container is provided by solid conduction through the container access conduit or neck tube. As is known in the art, the access conduit or neck tube penetrates the double-walled container's outer wall and inner vessel annd provides for ingress to and egress from the container's storage volume. The access conduit or neck tube also provides structural support for maintaining the container's inner vessel in a fixed spacial relationship with the outer wall or shell.
As double-walled cryogenic container insulations and their optimization as a system have approached very high efficiencies, the path of heat inflow by solid conduction through the neck tube has assumed greater significance in that it contributes a larger percentage of the total heat inflow from the ambient to the cryogen.
The prior art approach to minimize the heat inflow through the neck tube path has been generally to employ relatively elongated conduits. The purpose of the elongated conduit is primarily to increase the length of the path over which the inflowing heat must travel. However, this approach is limited by structural considerations. As the length of the access conduit or neck tube is increased to reduce heat leak, the thickness of the access conduit wall member must be increased in order to support the increased bending moment to which the access conduit is subjected to due to increased length. Increase of the access conduit wall member thickness will increase the heat transfer from the ambient into the stored cryogen by solid conduction through the neck tube or access conduit since this heat transfer mechanism is directly proportional to the cross-sectional area of the conduction path.
Accordingly, the art has been searching for an improved double-walled cryogenic liquid container access conduit design to decrease heat inflow from the ambient by solid conduction through the access conduit into a stored liquid cryogen.